1. Field of the Invention
The present invention relates to an electro-absorption modulator used in an optical communications system, and an optical semiconductor device wherein an electro-absorption modulator and a semiconductor laser are integrated on a semiconductor substrate.
2. Background Art
In an electro-absorption modulator, a separated confinement hetero-structure, wherein an optical waveguide layer having larger band gap energy than that of the light absorbing layer and a larger refraction index than that of the clad layer is formed on both sides of a light absorbing layer, is used.
In an electro-absorption modulator having an AlGaInAs light absorbing layer, a large valence region energy barrier is formed between an AlGaInAs light absorbing layer or an AlGaInAs optical waveguide layer and a p-type InP clad layer. This energy barrier becomes a resistor component when holes generated by light absorption in the light absorbing layer flow in the p-type InP clad layer. Therefore, hole pileup occurs, which worsens the flow of holes, and the dynamic extinction ratio, modulation bandwidth, and chirping characteristics of the electro-absorption modulator are worsened
A method for reducing the valence region energy barrier by the InGaAsP etching stopping layer formed between the AlInAs clad layer and the p-type InP clad layer has been proposed (for example, refer to Japanese Patent No. 3779040). However, the energy of the valence region cannot be sufficiently reduced by a single-layer InGaAsP layer. In addition, in the single-layer semiconductor laser based on AlGaInAs according to this background art, it is required to prevent the overflowing of electrons from the active layer into the p-type InP clad layer using an AlInAs layer. On the other hand, since the electro-absorption modulator is a reverse direction bias element, the overflow of electrons from the light absorbing layer to the p-type InP clad layer does not occur. Therefore, in the electro-absorption modulator, the use of the AlInAs layer is not required, and it is rather preferable not to use the AlInAs layer which has a high resistance and worsens the flow of the light absorbing current.